1. Field of the Invention
The present invention relates to a gloss applicator and an image forming apparatus including the gloss applicator. More specifically, the present invention relates to an image forming apparatus employing an electrophotographic method, such as a copier, a printer, a facsimile machine, or a multi-function apparatus having one or more capabilities of the above devices, and relates to a gloss applicator to apply glossiness to a toner image formed by any of the above-type image forming apparatuses.
2. Description of the Related Art
In an image forming apparatus using the electrophotographic method, a toner image is obtained using toner formed of thermally fusible resins and the like. The unfixed toner image transferred to a recording material (such as a recording medium, recording sheet, or simply a sheet) needs to be fixed thereon, so the image forming apparatus includes a fixing device for fixing the toner image onto the sheet with heat and pressure.
For example, a generally used fixing device employing a two-roller method includes a heat roller including a built-in heater and a pressure roller, the two rollers forming a nip portion to which the recording medium on which an unfixed toner image is transferred is conveyed and subjected to heat and pressure by the heat roller and the pressure roller to soften and fuse the toner of the unfixed toner image. The recording medium upon passing through the nip portion is then cooled and the toner is stiffened and fixed onto the surface of the recording medium. However, the toner image obtained through the above-configured fixing device does not show a sufficient glossiness, which has been a problem.
Various technologies have been proposed for a gloss applicator to make the toner image formed on the recording medium a high-quality image with an appropriate gloss during or after fixation by the fixing device. Thus, a fixing device including a gloss applicator or a fixing device capable of obtaining a high-quality print is disclosed in following patent documents: JP-2009-14876-A; JP-2004-325934-A; JP-H05-333643-A; and JP-2006-243444-A. Each of the disclosed fixing devices includes an endless belt stretched between a fixing roller and a support roller, and a pressure roller so disposed as to oppose the fixing roller, and further a gloss applicator to cool the recording sheet being in contact with the endless belt, after which the recording sheet is then separated from the endless belt to obtain a high-gloss print.
As a cooling means to cool the endless belt and the recording sheet that has been in contact with the endless belt, JP-2009-14876-A discloses a method to dispose cooling fans at an inner side of the endless belt and an external side of the endless belt to cool the belt by sending air from the cooling fan. The same also discloses a method to cool the endless belt by bringing it into contact with a heat pipe or a heatsink including water or other refrigerant.
Similarly, JP-2004-325934-A discloses a method to cool the endless belt by using a heat radiating member (such as a heatsink) to absorb and radiate heat by contacting an inner surface of the endless belt or by using a cooling device such as a cooling fan. JP-H05-333643-A discloses a method to cool the endless belt by disposing a cooling fan beneath the endless belt and by sending air from the cooling fan to the endless belt. The same discloses adjusting the air amount of the cooling fan.
Further, JP-2006-243444-A discloses a fixing device to heat a cooling member for the purpose of shortening a warm-up time. The disclosed apparatus includes a temperature adjusting mode in which the endless belt is controlled to rotate at a faster speed at start-up than during normal operation.
The gloss applicator of the belt type is configured such that the toner on the recording medium is once fused at a nip portion, the toner on the recording medium and the endless belt are pressed together, and the recording medium is conveyed and cooled in the contacting state and separated from the endless belt. As a result, the surface state of the endless belt is transferred to the surface of the toner so that the toner surface becomes smooth and a highly glossy print can be obtained. Accordingly, to obtain a high gloss image, the endless belt (that is, the toner surface) needs to be cooled to a desired temperature in a state in which the toner and the endless belt are closely contacted. Therefore, cooling property and cooling efficiency of the cooling means are important.
However, the above-described technologies do not sufficiently cool the endless belt to the desired temperature because an image forming apparatus with a higher printing speed in which the endless belt is driven at a higher speed does not have enough time to cool the belt.
In the cooling method using the cooling fan, because in the continuous printing the heater section of the heat roller of the endless belt radiates heat to increase an environmental temperature, the ventilation air is heated and the temperature of the belt gradually increases.
Even in the heatsink method to cool the endless belt while contacting it, because in the continuous printing the heater section of the heat roller of the endless belt radiates heat to increase an environmental temperature and causes the ventilation air to be heated, the temperature of the belt gradually increases so that the endless belt cannot be cooled to a desired temperature.
Further, in the heater section, the endless belt after having been cooled by the cooling means must be reheated to a predetermined temperature using only that portion of the belt that is wound around the fixing roller or the heating roller. However, if the cooling means is not sufficiently heated immediately after the initial activation of the machine or in a low temperature environment, the amount of electric power required for heating the heater section increases, as does the maximum electric power during printing. The power consumption of high-speed machines in particular increases.
To cope with this disadvantage, JP-2006-243444-A discloses an approach in which the endless belt is rotated faster than during normal operation and the cooling means is heated. However, achieving both good cooling and reduced power consumption simultaneously remains a challenge.